A patient's abdomen (alimentary canal) can be examined by X-ray after administering which one of the following metal salts in the patient's meal?

Barium sulphate (BaSO4) is the metal salt used as a radiographic contrast agent for examining the alimentary canal, often referred to as a 'Barium meal' [1]. Barium has a high atomic number (Z=56), which allows it to effectively absorb X-rays and delineate the structure of the gastrointestinal tract, including the esophagus, stomach, and intestines. While most barium compounds are highly toxic due to the Ba2+ ion, barium sulphate is safe for medical use because it is extremely insoluble in water and body fluids, preventing the toxic metal from being absorbed into the patient's bloodstream [2]. It is typically administered as a chalky white powder mixed with water or as a suspension. Other salts like barium chloride are soluble and toxic, making them unsuitable for ingestion.

Sources

1. [1] https://www.betterhealth.vic.gov.au/health/conditionsandtreatments/barium-tests
2. [2] https://www.ncbi.nlm.nih.gov/books/NBK598777/